There once was a saying in law enforcement that arson was the toughest crime to prove.

"A long time ago, the thought or the folklore was that the evidence was destroyed" by the flames, said Pennsylvania State Trooper Timothy Frew. "It's gone from an art to science. It's a purely science-driven forensic field."

Frew, 45, is retiring today from the state police, and for much of his 20-year career he sifted through ashes and rubble while investigating 427 fires in Indiana and Cambria counties.

Frew, who has been assigned to the Indiana PSP station for all but 1 1/2 years of his career, was working as a criminal investigator at the Somerset station when he became interested in fire investigation.

"The first death investigation I was assigned was a local police officer who was killed in his own home by fire," Frew said. "I was assigned the death part of that -- how he died. So the fire marshals came up from Greensburg to do the fire (part of the investigation). … They were there for three days. … They determined it to be arson. And they said, 'Tim, you now have a homicide.' And it just intrigued me. 'How did you do that? How did you figure that out?'

"That was my introduction," he said. "I always wanted to know how things worked. … That was the whole drive behind it."

Troopers like Frew are commonly referred to as fire marshals.

"I think that's a term from a long time ago," Frew said. "We're state troopers assigned to the fire investigative unit. We still carry the same duties as every other trooper but our primary responsibilities are fire and arson investigations."

A native of Jamestown, N.Y., Frew moved to the Apollo area while in middle school and graduated from Apollo-Ridge High School in 1985. He then served in the Marine Corps infantry for five years and "got to see the world two or three times." He participated in Operation Just Cause in Panama (which began 23 years ago today, with one goal being the capture of Gen. Manuel Noriega) and took part in the first Gulf War.

He joined the state police in March 1993 and became a criminal investigator just after 9/11.

State police fire investigators first serve a few years as part-time or alternate fire investigators under a station's full-time investigator. During that time as an alternate the trooper must complete four or five schools conducted by state and national fire academies and federal law enforcement agencies.

"They're pretty intense, with a lot of science," Frew said.

Once the schooling is completed and an alternate investigator has served enough time under a full-time investigator, he or she can apply when a position becomes open. That chance came for Frew in 2007.

PSP fire investigators are expected to uncover two things during a fire investigation -- first, the origin of the fire, then the cause.

"Ultimately, something, or somebody, is responsible. Fires just don't occur," Frew said. "If it's deemed to be accidental from a law enforcement standpoint, we're done." Private fire investigators, often working for insurance companies, may then take over and continue the investigation.

That is what's happening now at the Creps United Publications printing plant in White Township that was destroyed by fire in October. Private fire investigators, aided by engineers, are trying to determine exactly what was to blame for the blaze that caused an estimated $75 million loss.

Frew's work typically was initiated by fire departments.

"It's the fire chief's responsibility, at least in Pennsylvania, to determine what caused the fire," he said. "If they can't do that, we're the next phone call. So most of what we get called to is what they think is an intentionally set fire.

"Anybody can go into a fire that happened in a bedroom and figure it out. It's not difficult," Frew said. "But when it consumes the entire house it's a different kind of work."

Investigations can take days or months.

"At Creps, for example, it's probably going to go on till the end of February," he said.

When fire investigators arrive on the scene, they follow a protocol.

"The common practice is to start on the outside (of the building) and do a 360 walk-around," Frew said. "Photography is a big part of what we do. At the average house fire I take upwards of 800 pictures. … You start on the outside of the house and work your way from the least amount of damage to the interior of the house to the most amount of damage."

Documenting the heat-producing devices such as the electrical system and the furnace and interviewing people who were at the fire are also important parts of the process.

Sometimes heavy equipment is needed to dig down through the layers of debris to reach evidence.

"The thing most people don't know about a fire (investigation) is how labor-intensive it can be," he said. "I have spent days upon days shoveling on my hands and knees just to get to the floor of a kitchen. … You come out looking like a coal miner most days."

Another time-consuming part of his job is presenting his evidence in court.

"There's a two-week course that you have to sit through just learning how to testify as an expert," he said.

Frew has testified in 11 trials and countless preliminary hearings.

One of his most memorable fire investigations occurred in March 2007 when William Wheeler died in a fire at his Blairsville home. Frew ruled the fire was intentionally set.

Four months later, Wheeler's daughter, Codee Wheeler, then 17, was charged with homicide and arson.

During a five-day trial in 2008, private fire investigators testifying for the defense said a television malfunctioned and started the fire in the Wheeler home.

Codee Wheeler was acquitted.

That investigation, Frew said, was "life-changing" for him.

"It's my opinion nobody in the state police has more of a responsibility to what we're assigned to do," he said. "We change people's lives by our opinion. You can be ruined financially, you can go to jail for life, based on what I say happened at a fire scene."

That, he said, is an enormous responsibility.

"That fire was life-altering for me," Frew said, and made him realize he would sometimes have an arson defendant's life in his hands.

Frew lives in Home with his wife, Christina, and daughter, Brittany. His home will be the headquarters for the next chapter in his life. He has started his own private forensic fire investigation business, Fire Solutions. He'll continue probing through ruins for the causes and origins of fires, but now while under contract with insurance companies paying for fire losses.

"It's been a great 20 years," Frew said of his PSP career. "And it's been a blink of an eye."